1. Field of the Invention
The present invention relates to a position detecting apparatus for detecting the position of a position detection section pointed by a cordless position pointer and, more particularly, it relates to a position detecting apparatus including a position pointer provided with a tuning circuit and a position detection section provided with loop coils for transmitting electric wave by which a designated position of the position detection section can be detected by using the reflection of the electric wave from the position pointer.
2. Prior Art
A position detecting apparatus having a position detection section provided with a large number of signal lines arrayed at predetermined intervals both in the X-direction and in the Y-direction, and a position pointer for pointing a suitable measurement position of the position detection section, in which the position pointed by the position pointer is identified by catching the position as the X-Y co-ordinates by driving the X-direction and Y-direction signal lines successively, has been widely used as a graphic/character input apparatus generally called "tablet" or "digitizer".
As one of the apparatus of the type described above, a graphic information input apparatus is disclosed in Japanese Patent Unexamined Publication No. 58-176933 laid open on May 10, 1983. The graphic information input apparatus has a position detection section in which a plurality of driving lines are arranged at intervals in the X-direction and a plurality of detecting lines are arranged at intervals in the Y-direction and perpendicularly to the driving lines, and a position pointer having a magnetic material such as ferrite. In the disclosed apparatus, the driving lines are successively supplied with an electric current and the detecting lines are accessed sequentially so that the position pointed by the position pointer is identified by the position of the detecting line on which the highest voltage is induced.
The disclosed apparatus has an advantage in that the position pointer is easy to handle, because the position pointer can be provided as a cordless device having nothing but a function of transmitting a magnetic signal. The position of the position detection section which can be detected is a rectangular position surrounded by the driving and detecting lines. Accordingly, the resolution of the coordinates position is determined by the pitch of the lines. A higher resolution would be obtained by reducing the pitch of the lines, but such a reduced pitch will undesirably impair the signal-to-noise ratio, S/N, of the detection signal as well as the stability of detection performance, so that improvement in the resolution is limited by itself. In addition, it is rather difficult to detect the position right above each of intersections of the driving lines and the detecting lines. Furthermore, the disclosed apparatus has also a problem in that it is difficult to detect the position if the position pointer cannot be placed in the very close proximity of each driving line.
Therefore, the assignee of the present application has already proposed a position detecting apparatus as disclosed in European Patent Application Publication No. 0 259 894 dated Mar. 16, 1988 (corresponding to U.S. patent application Ser. No. 95,702 filed Sept. 14, 1987). The proposed apparatus has a position detection section in which a plurality of loop coils having rectangular patterns are arranged at predetermined intervals both in the X-direction and in the Y-direction, and a position pointer having a tuning circuit including a coil and a capacitor and adapted to resonate with electric wave produced by the loop coils. The proposed apparatus further has a selection circuit for successively selecting the loop coils one by one, a transmission circuit for generating an A.C. signal of a predetermined frequency to be supplied to the loop coils, a receiving circuit for detecting an induced voltage of a frequency substantially the same as that of the A.C. signal of the transmission circuit out of various voltages induced in the loop coils, a connection switching circuit for connecting the selected loop coil to the transmission circuit and the receiving circuit, and a controller means for determining the position pointed by the position pointer on the basis of the levels of voltages induced in the loop coils.
In the proposed position detecting apparatus, a loop coil is selected out of a plurality of loop coils by the selection circuit and the transmission circuit is connected to this selected loop coil by means of the connection switching circuit so that an A.C. current is supplied to the loop coil, by which an electric wave is generated by the loop coil. The electric wave thus generated excites the coil in the position pointer which points a position on the position detection area, so that a voltage is induced in the coil in synchronization with the A.C. current. If the connection switching circuit is changed over from the transmission circuit side to the receiving circuit side, the electric wave from the selected loop coil will stop. Thereafter, the receiving circuit is connected to the loop coil selected by the connection switching circuit and, at the same time, the supply of the A.C. current is ceased so that the electric wave is extinguished. Meanwhile, a current corresponding to the induced voltage flows in the tuning circuit in the position pointer, so that the coil in the tuning circuit generates an electric wave. This electric wave reversely excites the loop coil which has been selected and connected to the receiving circuit, so that a voltage is induced in the loop coil. The aforementioned change-over between the electric-wave transmission and reception is conducted successively on all the loop coils. Because this change-over between the electric-wave transmission and reception relies upon resonance between each loop coil and the coil in the position pointer, the level of the voltage induced in the loop coil has such a dependency on the distance between the loop coil and the coil of the position pointer that the level of the induced voltage is highest in the loop coil which is closest to the present position pointed by the position pointer and is gradually decreased as the distance between the loop coil and the coil of the position detector is increased. The levels of voltages induced in these loop coils are processed by the controller means so that the coordinates of the position pointed by the position pointer can be identified by the position of the loop coil which exhibits the highest level of the induced voltage among the loop coils. Because the position pointed by the position pointer is determined by the induced voltage thus detected, the precision of position detection can be increased by enhancing the precision of arithmetic operation of the controller means on the levels of induced voltages in the plurality of loop coils. Thus, a high-resolution position detecting apparatus can be provided.